FIG. 8 is a cross-sectional view of a conventional image intensifier tube, generally designated as 80. As shown, image intensifier tube 80 includes a fiber optic cathode plate, generally designated as 81, microchannel plate (MCP) 82, and a fiber optic anode plate, generally designated as 83. Light enters fiber optic cathode plate 81 and is guided through multiple fibers 85 striking a GaAs photocathode 89, that is bonded to surface 87 of the fiber optic cathode plate. Due to photoelectric conversion, electrons are emitted from photocathode 89. The electrons exiting from fiber optic cathode plate 81 are amplified by MCP 82. These electrons are accelerated and caused to impinge on phosphor face 84 of fiber optic anode plate 83, thereby emitting fluorescent light. The emitted light is guided through fiber optic anode plate 83, by way of multiple fibers 86, so as to yield output light.
Although not shown, the exiting light from fiber optic anode plate 83 may be coupled to a charge coupled device (CCD) by way of a tapered fiber optic coupler. As shown in FIG. 8, output surface 87, MCP 82 and phosphor face 84 are contained within a vacuum formed by housing 88.
It will be appreciated that image intensifier tube 80 is not drawn to scale. More specifically, multiple fibers 85 and multiple fibers 86, respectively, in the fiber optic cathode plate and the fiber optic anode plate are not drawn to scale. There are typically millions of fibers 85 and 86, in these cathode and anode plates.
As shown in FIG. 9, each of multiple fibers 85 and 86 include fiber 90, which may or may not be of the same materials or dimensions. Fiber 90 includes glass rod 92 and glass cladding 91, which surrounds the glass rod. The glass material of cladding 91 is different from the glass material of rod 92.
Optical fiber 90 is formed in the following manner. A glass rod and a cladding tube, coaxially surrounding the glass rod, are suspended vertically in a furnace. The temperature of the furnace is elevated to the softening temperature of the glass. The rod and cladding tube fuse together into single fiber 90. Fiber 90 is fed into a traction mechanism, where the speed is adjusted until a desired fiber diameter is achieved. The fiber is then cut into shorter lengths.
Several thousands of the cut single fiber 90 are then stacked into a mold and heated to a softening temperature of the glass, in order to form an array 100, as shown in FIG. 10. Array 100 is also known as a multi assembly or a bundle and includes several thousand single fibers 90, each having a rod and a cladding. The multi assembly 100 is suspended vertically in a drawing machine and drawn to decrease the fiber diameter, while still maintaining the configuration of the individual fibers. The multi assembly 100 is then cut into shorter lengths of bundles.
Several hundreds of the cut bundles 100 are then stacked and packed together into a large diameter glass tube (not shown). After stacking and packing the bundles, the entire assembly is heated and fused together. In this manner, active areas of the fiber optic cathode plate and the fiber optic anode plate are formed from the millions of individual fibers 90.
Strong light energy entering a fiber optic faceplate, such as fiber optic cathode plate 81 or fiber optic anode plate 83, has been known to damage the image intensifier tube. Any damage to the gallium arsenide (GaAs) photocathode 89 or any other downstream components caused by strong laser light produces a permanent black spot on the image tube.
Attempts have been made to create an absorber cladding in the fiber optic bundle. These efforts have been focused on drawing a fiber that contains an absorber within the cladding materials. It has been conjectured that these absorbers in the fiber cladding may enhance the effectiveness of a fiber optic faceplate in limiting laser induced damage to the GaAs deposited on the vacuum side of the fiber optic faceplate. Other efforts have attempted to use cladding materials in the fiber optic bundle that selectively blackens when exposed to hydrogen gas.
The efforts for producing a fiber optic bundle with absorbing cladding material requires several steps. First, a fiber having an absorbing or other reactive cladding must be placed in a drawing machine and drawn to a desired diameter size. Each of these fibers must then be fused into a bundle that may contain over 1 million fibers. The bundle must next be fused to a GaAs wafer (for example) to form the photocathode.
To the best of the inventor's knowledge, the above efforts have not been attempted, since they likely take significant investments to overcome known and unknown material and process incompatibilities to eventually form a modified fiber optic faceplate. In addition, the materials in the modified fiber optic faceplate must be compatible with image tube assembly, sealing and activation processes, which have required decades to stabilize in order to achieve today's high performance and reliability levels.
What is needed is a method for making a limiter device for fiber optic faceplate night vision goggles which limits any damage from strong light entering the image intensifier tube. In addition, the modified fiber optic faceplate materials must be compatible with conventional image tube assembly, activation and sealing processes. This invention addresses such a need.